hacer que la opción Seleccionar se vea como un botón

/* HTML is here */

<select name="work_days" id="id_work_days" multiple>
  <option value="1">sun</option>
  <option value="2">mon</option>
  <option value="3">tue</option>
  <option value="4">wed</option>
  <option value="5">thu</option>
  <option value="6">fri</option>
  <option value="7">sat</option>
</select>



/* CSS is here */

#id_work_days{
  height: 44px;
  border: none;
  overflow: hidden;
}
#id_work_days::-moz-focus-inner {
  border: 0;
}
#id_work_days:focus {
  outline: none;
}
#id_work_days option{
  width: 60px;
  font-size: 1.2em;
  padding: 10px 0;
  text-align: center;
  margin-right: 20px;
  display: inline-block;
  cursor: pointer;
  border:rgb(204, 204, 0) solid 1px;
  border-radius: 5px;
  color: rgb(204, 204, 0);
}
Beautiful Bear